The present invention relates to an automatic focusing method which can be suitably applied to a device having an automatic focusing capability function, such as an AF (automatic focusing) camera.
Recently, a variety of lens-interchangeable cameras or video cameras having an AF function have become commercially available. With these cameras, it is possible for anyone to readily achieve photographing operations or video recording operations as desired.
By way of example, convention automatic focusing function will be described with reference to a lens-interchangeable camera.
First, the general arrangement of an automatic focusing camera having an automatic focusing function (hereinafter referred to as "an AF camera" when applicable) will be described with reference to FIG. 1.
In FIG. 1, reference numeral 11 designates a photographing lens, and 31, a camera body on which the photographing lens 11 is mounted.
The photographing lens 11 includes a lens system 15 including a lens 13 which is freely movable in the direction of the optical axis for focusing; a drive force transmitting mechanism 17 composed of a clutch 17a, a gear 17b and a helicoid screw for transmitting a drive force from a drive mechanism 47 (described below) in the camera body 31 to move the lens 13 to a focusing position; a lens ROM (read-only memory) 19 for storing aperture value data for the photographing lens and position data for the lens 13; and an electrical contact group 21 on the lens side for electrically connecting the photographing lens 11 to the camera body 31.
The camera body 31 includes an optical system composed of a main mirror 33, an auxiliary mirror 35, a focusing screen 37, a pentagonal prism 39, light detecting element 41a and 41b used for AE (automatic exposure) control, and an electrical contact group 43 on the body side which is coupled to the electrical contact group 21 on the lens side. The camera body further includes an image pick-up section 45 which receives a part of the light beam from an object to be photographed, which has passed through the lens system 15 to form the image of the object. In the case where the focusing position detection is carried out according to the correlation method (phase difference system), the image pickup section 45 is implemented with an optical system including a separator lens, and a CCD (charge-coupled device) having two image pickup regions. The camera body 31 further includes the drive mechanism 47 for driving the movable lens 13 in the photographing lens 11. The drive mechanism 47 has an AF motor 47a which is a DC motor, a gear 47b, a clutch 47c and an encoder 47d for controlling the speed of the motor 47a. The drive force of the drive mechanism 47 is transmitted through the gear 47b, the clutch 47c of the camera body 31 and the clutch 17a of the lens 11 to the movable lens 13, whereby the lens 13 is moved along the optical axis. The camera body further includes a control section 49 which operates to calculate an amount of defocusing D indicating the amount of deviation from the focused position in response to the output signals of the image pickup section 45, the value D being employed to drive the drive mechanism 47 according to the amount of defocusing D, and to perform automatic exposure photographing operations.
In the camera having an AF capability as described above, the automatic focusing operation to move the movable lens 13 to the focused position is carried out as follows:
First, a shutter button (not shown) is depressed halfway, or a focus lock button (not shown) is depressed, whereby the camera is placed in the automatic focusing mode. In this case, the control section 49 uses the output data of the image pickup section 45 to obtain defocusing data including an amount of defocusing D and a direction of defocusing for the object according to the conventional correlation method. Thereafter, the control section 49 determines, according to the amount of defocusing D, the amount of drive P required for the motor 47a to move the movable lens 13 to the focused position.
The amount of drive P can be determined in terms of a number of rotations of the motor 47a; that is, it can be set according to the pulse count number of the pulse signal from the encoder 47d which detects the rotation of the motor. For this purpose, in the control section, the pulse number P for eliminating defocusing is obtained according to the following equation (1): EQU P=K.multidot.D . . . (1)
where K is the lens movement conversion coefficient. The coefficient K is predetermined so as to obtain a pulse count number P which moves the movable lens 13 to the position where the lens is positively focused on the object according the value D. Each individual photographing lens has its own coefficient K. The value K is stored in the lens ROM 19.
The motor 47a is driven until the total number of pulses of the pulse signal from the encoder 47 reaches the value P in equation (1). At that point, the object will have been automatically focused.
When the shutter is operated or the focus lock button is released, the automatic focusing operation is accomplished.
In this conventional automatic focusing method, focusing is carried out only in response to light from a designated physical object. In other words, the lens can be focused only on a single physical object, and it is impossible to focus the lens at a spatial point between one physical object and another physical object.
This causes the following difficulties.
The case will be considered where it is required to photograph a plurality of objects located at different positions in one picture. In this case, it is preferable that the lens be focused on a point located between these objects and a certain distance away from the camera so that the lens is substantially equally focused on all the objects. However, the conventional automatic focusing method cannot do this.
Furthermore, if, in the case where only one object is to be photographed and the object has upwardly and downwardly curved parts, it is preferable that the photographing operation be carried out with the lens focused on a point between the curved parts from the camera. However, the conventional automatic focusing method again cannot do so.
When the shutter is operated once or the focus lock button is released, the focusing operation is ended, and therefore a series of focusing steps must be carried out when another picture is to be taken. Therefore the conventional focusing method suffers for a difficulty in that it is impossible to carry out photographing operations successively at one and the same focusing position.